Two-stroke internal combustion engine



Sept; 1938. M. KADENACY 2,130,721

TWO-STROKE INTERNAL COMBUSTION ENGINE Filed June 8, 1936 8 Sheets-Sheet l no to Q 2 800 RP. M.

aanssaad F000 R.P.M.

PRESSURE E j nvenzor: Katie/229v Sept. 20, 193'8. -M. KADENACY 2,130,721

TWO-STROKE INTERNAL COMBUSTION ENGINE Filed June 8, 1956 8 Sheets-Sheet 2 IZOORPM.

RE'ssuRE g: d w

: 2' u Jul aanscaua jOh Mrw Sept. 20, 1938. M. KADENACY 2,130,721

TWO-STROKE INTERNAL COMBUSTION ENGI NE Filed June 8, 1936 s Sheets-Sheet 5 SepL'ZO, 1938. M. KADENACY" I 2,130,721

TWO-STROKE INTERNAL COMBUSTION ENGINE Filed June s, 1936 s Sheets-Sheet 4 :l\ t 'a'crq Y aan saad j fl'en (or: Weme P 1938. M. KADENACY 2,130,721

TWO- STROKE INTERNAL I COMBUSTION ENGINE Filed June 8, 19,56 8 Sheets-Sheet 5 {2 l5 3 00' o Q @013 Q 2 E 3' ("g ldl B.D.C.

jH/ehfar. Kids/m9 WZ 1M p 1938. M. KADENACY 2,130,721

TWO-STROKE INTERNAL COMBUSTION ENGINE File ne 6 8 Sheets-Sheet 6 PORT AREA El C.MS.

ATM 0 40 30 20 I0 10 20 30 W f M? p 1938. M. KADENACY 2,130,721

TWO-STROKE INTERNAL COMBUSTION ENGINE Filed June 8, 1936 8 Sheets-Sheet 7 WjWM Sept. 20, 1938. I MKADENACY 2, 30 12 TWO-STROKE INTERNAL COMBUSTION ENGINE Filed June 8, 1936 8 Sheets-Sheet 8 6 F 1 W F 1 HK LM Patented Sept.20,1938 2,130,721

TWO-STROKE INTERNAL COMBUSTION ENGINE Michel Kadenacy, Paris, France, assignor of onehalf to Armstrong Whitworth Securities Company Limited, London, England Application June 8, 1936, Serial No. 84,182 In Great Britain January 11, 1936 4 Claims. (Cl. 12365) This invention relates to a method of conthe cylinder will be left in a highly rarefied constructing two-stroke cycle internal combustion dition. engines of the kind wherein at least a substantial The void or depression left behind the mass of portion of the burnt gases leaves the cylinder at burnt gases exists not only in the cylinder, but 5 a speed much higher than that obtaining when also in the space into which the burnt gases issue, 6 an adiabatic flow only is involved, and in such e. g., the exhaust duct or system, and the length a short interval of time that it is discharged as a of time this void or depression is maintained is mass leaving a depression behind it which is determined by a return of the rear zone of the utilized in introducing a fresh charge into the gaseous mass on its return rebound towards the 10 cylinder. cylinder.

The object of the invention is to provide a It will be understood that if a portion of the method of constructing an improved engine of ballistic energy of the burnt gases is destroyed the above kind. during their mass exit from the cylinder the The invention comprises selecting a crank anvolume of the void left behind the gases and the gle for the charging period, establishingthe area duration of time for which this void is mainof the inlet orifice and the rate of opening of tained will be correspondingly reduced. this orifice so that a working charge can enter by Another object of the invention is therefore to atmospheric pressure during the charging period provide a two-stroke internal combustion engine as a consequence of the mass exit of the burnt as set forth above, in which the burnt gases regases from the cylinder, selecting a moment for tain the maximum proportion of their ballistic 20 opening the exhaust orifice, and making the energy when they leave the cylinder, and in which area of the exhaust orifice and the rate of opena maximum utilization is made of the void left ing of this orifice such that the area of exhaust by the burnt gases upon their mass exit from the orifice opened before the opening of inlet ensures y de that the mass exit of the burnt gases occurs be- With this object in view, the invention further 2 fore the opening of inlet. consists in selecting a crank angle for the charg- The present inventor has already indicated in ns p ri d, i s s n a maximum area of prior specifications that the behavior of the gases the inlet orifices and a maximum rate of ope in upon and after their discharge from the cylinder o these fi W t mechanical limits,

is such as to lead to the belief that the burnt tablis in exhaust ifices havin a maximum 30 gases while still in the cylinder form a, body area withinmechanical limits and establishing having properties similar to those of a resilient the Pe d between the opening of the in dy, and which upon the opening of an exhau t orifices and opening of the exhaust orifices such orifice seeks to project itsgLf a a mas fr m th that the inlet orifices will open when the burnt cylinder. He has observed that when th gases commence to leave the cylinderas a mass 35 haust orifice opens there is first a period of delay and arranging for the area 0f exhaust orifices during which the gases do not emerge from the which is opened prior to the opening of the inlet cylinder, and that after this delay has elapsed fi s to be opened at a maximum rate within the burnt gases issue from the cylinder at a, speed mechanical imits, whereby the maximum char e greatly in excess of the speed obtaining for an may be introduced into the cy inde in the time 40 adiabatic flow and as a mass the motion of which availableis governed by the laws of reflection and rebound. The invention also 9 m pmvldmg means This high velocitywill hereinafter be referred muting any b1et1nab1e Influence to as the ballistic speed of the burnt gases, and return of the burnt gases. fi g 8110' the force causing this high velocity will be reon the charge contam? m 6 cy m ferred to as the ballistic energy of the burnt gases. Further featut'es of t i m appear from the following description in which refer- In engines of the type to which the present ence will be made to the attached drawings. invention relates the whole or at least a substan- In the drawings,

tial portion of the burnt gases leaves the cylin- Figures 1 to 5 are records f pressures taken 50 der ballisticallyas a mass. If all the exhaust during the exhaust d inlet'periods in t gases leave ballistically as a mass the cylinder haust pipe of an engine according t th will be left substantially void of burnt gases. If v ti close t t cylinder, th figures only a substantial portion of the exhaust gases sponding respectively to speeds of 800, 1000, 1200,

leaves ballistically as a mass the burnt gases in 1300 and 1500 R. P. M. 55

Figure 6 shows three records of pressures taken on the exhaust duct of another engine, at one speed but with different exhaust systems on the engine.

Figures 7, 8 and 9 are records of pressures taken on the inlet duct of an engine during the exhaust and admission periods.

Figure 10 is an exhaust and inlet port area diagram.

Figures 11 and 12 show by way of example a cross section through a cylinder of an internal combustion engine showing the exhaust ports and the passages leading from these ports into an exhaust pipe.

Figure 13 is a timing diagram. relating to a sixcylinder engine.

Figures 14 and 15 show an arrangement of exhaust duct and manifolds in accordance with the invention for a six-cylinder engine.

The records shown in Figures 1-9 were all obtained by means of the device described in application Serial No. 82,958 filed June 1, 1936.

In these figures the ordinates represent pressures above and below atmospheric pressure and the abscissae represent crank angles. EO, EC, AC, AC, indicate respectively exhaust opening and closing and inlet opening and closing, while bottom dead centre is indicated at BDC.

The curves shown in Figures 1 to are differential pressure curves which also enable the velocity and direction of movement of the gases to be observed.

Each of these figures shows two similar curves, one in full lines and one in dotted lines. Where the full line curve lies above the dotted line curve,. the direction of motion is away from the cylinder.

The difference between the ordinates of the two curves at any point is also an indication of the velocity of the gases at any moment.

By referring to any one of Figures 1 to 5, it will be seen that when the exhaust port opens there is first a delay during which substantially no change in pressure occurs in the exhaust pipe and that thereafter the pressure rises abruptly to a peak, then falls abruptly to a pressure below atmospheric pressure and that the pressure sub sequently rises again and destroys the depression.

. If these changes in pressure are considered in conjunction with the direction of motion of the gases, it will be seen that during the period of delay, a substantially constant and very small pressure difference exists in the exhaust pipe. Thereafter the pressure difierence increases very rapidly to a maximum pressure difference which remains substantially constant for a little distance on either side of the pressure peak of the curve. This can be taken as representing the passage of the mass of burnt gases at high velocity past the indicating device.

Thereafter the movement continues to be in the direction away from the cylinder, but by this time the mass of burnt gases has passed the indicator and is at some point further along the exhaust pipe and the pressure difference indicates the state of the medium which is at that moment passing the indicator.

It will then be seen that at a later point in the crank angle, the two curves cross indicating a reversal in the direction of motion and that this reversal is followed by the return impact of the burnt gases.

The curves shown in Figures 1 to 5 were obtained from an engine constructed in accordance with the invention. It will be seen that inl t opens substantially when the tail end of the mass of burnt gases has left or is about to leave the cylinder, and it will also be seen from the curves that the entering charge arrests the rapid fall in pressure and partly fills the void left behind the mass of burnt gases. Subsequently the pressure again falls in the exhaust pipe before the reversal in direction occurs, showing that in this example the entering charge cannot fill completely the void left by the burnt gases in the exhaust system.

If the inlet orifice instead of being opened in the manner indicated above had remained closed when the burnt gases left the cylinder, the curves would have shown a fall in pressure to a very high degree of vacuum and an equally abrupt return to a high pressure, since there would be nothing in the cylinder to oppose or attenuate this return, and it should be understood from the above that the form of the curve is modified by the fact that the entering charge follows the issuing burnt gases, although the characteristics I of the curve remain unaltered.

From the curves it can be seen that the rise in pressure to the peak and the subsequent drop in pressure are extremely abrupt and cannot be compared with an adiabatic expansion and that the total exhaust occupies an interval of time which is shorter than that which would be required for the burnt gases to expand adiabatically down to atmospheric pressure, and it can be concluded that the true and instantaneous speed of mass exit from the cylinder is much higher than the mean overall speed of exit and is higher than the speed of sound.

In this connection it should be noted that the indicator records changes of pressure in the exhaust pipe close to the cylinder. If the accompanying changes in pressure in the cylinder are considered it will be understood that the abrupt rise in pressure tothe peak of the curve corresponds to an equally abrupt drop in pressure in the cylinder behind the gases.

Further, it can be shown by multiple observations that after the exhaust the cylinder remains under a depression and it will be seen from the curves that this depression is prolonged for a certain duration of time, after which the return impact occurs.

From the above observations it can be concluded that the gases before their exit from the cylinder have an initial velocity proper and that adiabatic expansion only intervenes as a secondary phenomenon.

The above and other observations have led the inventor to conclude that the mean speed of ballistic exit of the burnt gases from the cylinder is of the order of 1400 to 1800 metres per second, taking into account the fact that this exit does not occupy the total time interval elapsing between exhaust and inlet opening, in view of the initial delay that must exist and the tolerance that must be allowed in a variable speed engine.

The gaseous medium external to the exhaust orifice possesses a static inertia that has to be overcome by the burnt gases before the latter can issue from the cylinder.

But when the exhaust orifice first commences to open and only a narrow slit is open, the mass of burnt gases cannot on account of its viscosity emerge from the cylinder and engage fully with the gaseous medium external to the exhaust orifice,.and so is unable to exert its full force on this external gaseous medium.

When such a sufllcient area of exhaust orifice has opened, the burnt gases may not at this moment be directed towards the exhaust orifice, in other words, there may be a delay due to the dynamic inertia of the burnt gases.

The truemoment of commencement of exhaust will be when a sufficient area of exhaust orifice has opened for the burnt gases to emerge bodily from the cylinder and the said gases issue from the cylinder as a mass and overcome the resist ance of the external gaseous medium.

The above effects explain the period of delay that occurs at the commencement of the curves shown in Figures 1' to before the abrupt rise If the angle between exhaust opening and inlet opening has been determined in advance it will be understood that a sufficient area of exhaust orifice must open before inlet opens to ensure that the mass exit of the burnt gases from the cylinder occurs within this angle, and this may i be arrived at from a consideration of the mean velocity of total exit of the burnt gases referred to above and of the mean area of the exhaust orifice opened in the angle in question and the fact that the volume of burnt gases must be discharged in an interval t of time which is shorter than that required for the adiabatic expansion to manifest itself as a dominating factor.

For the purpose of making such a calculation the time interval t within which the total exhaust must be effected may be'taken as being of the order of 1/300 sec. It may of course with advantage be smaller than this amount.

The inventor has found that calculations of suflicient accuracy to ensure practical results may be made by assuming that the cylinder volume of burnt gases is discharged, without expansion, at a hypothetical mean speed. This hypothetical mean speed of discharge will vary according to the fuel employed, the mixture and the conditions of combustion, among other fac- N is the number of revolutions per second of the engine A isthe area of exhaust orifice opened before inlet opens in cm K is a constant depending upon the form of exhaust orifice and the area opened per unit movement of the piston or crankshaft Then the length of the column formed by the passage of the mass of burnt gases through the exhaust orifice will be t IOOKA me The time interval occupied by this mass exit will IOOKA, The time elapsing between exhaust opening and inlet opening will be 8 W QCCS.

so that the following relationship should exist But it will be seen that in order to reduce the period of total exit of the burnt gases to a minimum the area of the exhaust orifice should be made as large as possible, and the area of exhaust orifice which is opened prior to the opening of the inlet orifice should be opened at a maximum rate, all within mechanical limits. In this case it will be more simple to open the full area of the exhaust orifice at a maximum rate but it will be understood that the rate of opening the remaining area of the exhaust orifice after the gases have left the cylinder is not of importance.

In this way if the angle available for evacuating and charging the cylinder has been determined in advance, the maximum portion of this angle willbe available for the purpose of charging.

0 The choice of such an angle will depend upon the construction of the engine and will be determined in accordance with well-known principles in order to give the maximum useful cylinder volume.

By experiment it has been found by the inventor that the passage to the atmosphere can be opened when the outgoing gases have left the place at which the admission orifice is situated. For example in an opposed piston engine wherein the inlet and exhaust orifices are at opposite ends of the cylinder, the admission orifices can be opened when the exhaust gases are still in the bottom of the'cylinder, near the place at which the exhaust orifices are situated; this is explicable by the viscosity of the gases and by the velocity with which they are leaving.

From the foregoing, and as described in application Serial No. 738,014 filed August 1st, 1934, it will be seen that if the inlet is opened at the moment when the issuance of the burnt gases as a mass is in' full progress and causes a suction effect to be exerted in the cylinder 9. fresh charge may be admitted by the action of the ambient atmospheric pressure. More precisely, inlet should open when the rear face of the issuing mass of burnt gases leaves the exact position of the inlet orifices.

If the opening of inlet is delayed, the time available for charging is reduced, and the return impact occurs more quickly and with greater violence. The curves show that the latest moment at which such an opening could be effected is immediately before the reversal in direction of motion occurs.

In order to obtain the maximum result inlet should open without delay and with the greatest area opened per unit of movement of the piston. But in practice a fairly large tolerance is permissible from the constructional point of view in order to obtain a highlysatisfactory operation of the engine.

It is therefore possible to choose. a moment for opening the inlet orifice or orifices which is situated relatively close to the moment of exhaust of the burnt gases and which remains suitable for charging the engine over a wide range of speeds.

In Figures 1 to 5 it will be seen that the timing of inlet opening remains unaltered and that over the range of speeds represented by the curves, this opening of inlet is always relatively close to the moment of exit of the burnt gases.

For example in Figures 1 to 5 inlet is timed to open 28% after exhaust and this timing remains suitable over at least the speed range represented by the figures. The volume of the en gine cylinder is 700 ccs. and the area of exhaust orifice opened before inlet opens is 13.8 sq. cms. If the rate of opening of the exhaust orifices is increased the inlet may be opened 20 after exhaust when 13.2 sq. cms. of exhaust orifice have opened. The maximum rate of opening that is mechanically possible in this engine will permit inlet to be opened 18 after exhaust when 13.6

- sq. cms. of exhaust orifice have opened. In all three cases the total area of exhaust orifice is larger than that open at the moment of inlet opening, and approaches the maximum area that can be allowed.

In order that the cylinder may be filled with a fresh charge, the inlet orifices must have a sufficiently large area and be open for a sufiiciently long time in order that a fresh charge can enter under atmospheric pressure into the cylinder and into a part of the exhaust duct.

If the angle available for charging has been determined, the area of the inlet orifice that must be opened to ensure the entry of a suificient charge may easily be determined from a consideration of the known mean speed of expansion of air from atmospheric pressure into a void and of the volume of the void to be filled.

For example a calculation may be based on the requirement of introducing say 1.5 cylinder volumes of air into the cylinder and exhaust duct, and a mean speed of entry of the charge of 50 to 60 metres per second maybe assumed. This value will be a conservative one and the mean speed of entry for practical purposes may be as high as 100 metres per second.

It will however be preferable and also more simple in practice if the inlet orifice is made as large as possible and the full area of the inlet orifice is opened as rapidly as possible so that this full area will be available for charging for the longest possible portion of the charging period.

The curves show clearly the time necessary and available for the introduction of the charge from which a. suitable admission arrangement can be arrived at and how the area and time of opening of the inlet orifice may be established in relation with the characteristics of the exhaust system.

(1) The portion e of the exhaust curve up to the moment when the inlet orifice opens. This area must be suiiicient for the'purpose set forth and preferably the slope of the curve should be as steep as possible.

(2) The portions of the curves. I and 2 that overlap. During this angle both inlet and exhaust are open, and air is passing through the cylinder and the exhaust duct. A variation in this angle will vary the amount of cooling obtained by the air that passes through the cylinder.

(3) The portion a of the admission curve, after the exhaust has closed and when the pressure in the cylinder is restored to the ambient pressure. The velocity of the incoming charge will during this period add its action to that due to a difference in pressures.

Now the charge introduced must be retained in the cylinder without being forced out or sucked out by the movements of the exhaust gases which either continue in the direction leading from the cylinder or are transformed into a return impact by the rebound which occurs either in the exhaust ducts or in the open atmosphere.

The curves shown in Figures '7, 8 and 9 indicate the manner in which the reversal in direction of the burnt gases is transmitted through the inlet. In Figure 7 the full and dotted line curves serve to show, as in Figures 1 to 5, the moment when this reversal in direction appears at the inlet.

From these three figures it will be seen that at low speeds the return impact of the gases is liable to be objectionable and that at high speeds the inlet may close before the return occurs.

The return impacts force out and foul the charge; an unnecessarily prolonged suction which follows the admission reduces the charge by placing it under a depression. These two objectionable factors come from outside the cylinder and have a repercussion upon its contents. They must either be suppressed or attenuated, or retarded or separated from the cylinder, so that their influence cannot affect the contents of the cylinder itself. This may be obtained by the timing of the engine itself, by the timing of the exhaust proper, by an arrangement of the exhaust system in form and in volume or by corresponding arrangements in the exhaust system.

For example a suitable exhaust closure may be established as described for example in application Serial No. 84,184 filed June 8, 1936, or in application Serial No. 83,120 filed June 2, 1936, whereby the objectionable factors in question may be separated from the cylinder.

Or the means described in application Serial No. 738,016 filed August 1, 1934 may be utilized in order to prevent the return impact from reentering the cylinder.

The introduction of a supplementary compressed charge at the end of the admission period as described in application Serial No. 745,814 filed September 27, 1934 or the injection of air into the exhaust duct at a suitable moment as described in application Serial No. 46,804 filed October 24, 1935 will oppose and retard the return impact.

The use of means such as those described in application. Serial No. 38,826 filed August 31, 1935 and in application Serial No. 82,959 filed June 1, 1936 and those described in application Serial No. 46,805 filed October 25, 1935, will permit the tenuated.

These means are simply indicated by way of example and any means which ensure that the charge will be retained in the cylinder may be employed.

The inventor has found by experiment that the absolute velocity of exit of the burnt gases from the cylinder may be retarded or accelerated according to the nature of the space which the gases enter when they leave the cylinder.

For example exhaust pipes which are of too large dimensions in cross sectional area retard the speed of exit and bring the return impact nearer. Tubes which are too small in cross sectional area retard the'exit by compressing the column and deforming the gaseous body; the increase in density of the burnt gases as a consequence of this compression causes the column of exhaust gases to lose momentum, due to the consequently increased friction. In all these cases it can be understood that the actions referred to are those which produce a negative acceleration on the issuing gases. Tubes of suitable dimensions maintain the speed of the gases, and the total exit of the gases from the cylinder occurs more rapidly.

The exit of the exhaust gases directly into the atmosphere, that is, when no exhaust system is present, occurs with a high loss in velocity; the return impact by rebound follows immediately and the duration of time for which the cylinder remains void is shorter than in all the other cases.

A further object of the invention is to indicate the requirements that must be fulfilled by an exhaust pipe for an engine constructed in accordance with the invention, in order to ensure that the issuing mass of burnt gases will be subjected to a minimum deceleration during its outward motion.

As stated above when the exhaust gases leavethe engine cylinder consequent upon the opening of the exhaust ports, they tend to form a column, the length of which will be dependent upon the area of the exhaust orifices open during the mass exit of the gases.

At this moment the issuing mass of burnt gases possesses a very high velocity and conform to the laws of reflection.

Consequently upon issuing from the cylinder it should encounter in the exhaust passages and in the exhaust pipe no surfaces capable of reflecting it back into the cylinder or of impeding its motion away from the cylinder.

Further, the energy contained in the exhaust gases is capable of displacing a proportionate mass of the resisting gaseous medium external to the cylinder. If this resisting medium presents a large surface to the issuing mass of burnt gases, the latter will be deformed and flattened, and the issuing column will be of shorterlength, its negative acceleration will be greater, and its return to the cylinder will be more rapid.

Exhaust pipes of too large a diameter relative to the exhaust ports will cause this to occur, and the extreme case will be encountered, if the exhaust gases are allowed to issue directly into the open atmosphere.

On the other hand, if the exhaust pipes are of too small a diameter, the resistance to deforma tion of the issuing mass, which may possess a 7 very high viscosity again exerts too great a decelerating motion on the issuing mass. It re- .return impact.

tards the complete evacuation of the burnt gases and causes the return to occur more rapidly.

In both cases therefore the time available for effecting the admission is reduced.

According to the invention the walls of the passages and ducts through which the burnt gases pass upon leaving the exhaust orifices are so formed that they always tend to guide and reflect the burnt gases away from the cylinder in the direction of exhaust and the section of passage for the burnt gases through these passages and ducts is made such that the burnt gases upon leaving the exhaust orifices do not thereafter encounter any sudden and considerable increase or decrease in cross section capable of causing an increase or decrease in cross sectional area of the column.

Preferably in order to facilitate the outward movement of the burnt gases and hinder their return towards the cylinder, the exhaust ducts will be increased progressively in cross section in the direction of exhaust.

This progressive increase in cross section will allow for the expansion of the burnt gases during their motion away from the cylinder.

A suitable arrangement for carrying out the above requirements is indicated in Figures 11 and 12. In these figures it will be-seen that the passage 4 in the cylinder block connecting the exhaust orifices 3 with the inlet end of the exhaust pipe- 5 is so formed that it comprises no surfaces capable of reflecting the issuing gases back into the cylinder and that from its point of connection with the cylinder the exhaust pipe increases progressively in cross section.

In arranging the form'and shaped the exhaust passages, account should be taken of the fact that the exhaust mass tends to be projected from the cylinder in a natural direction which is that of the cylinder axis, and it is for this reason that in Figure 12 the exhaust passage is inclined tothe axis of the cylinder in order to deflect the issuing mass of burnt gases as little as possible.

A further object of the invention is to specify the relationship that should exist between the length of the exhaust pipe and the return impact of the burnt gases for the most advantageous operation of the engine.

It has been stated above that the inventor has found by experiment that when the burnt gases .issue from the cylinder directly into the atmosphere, the return impact of the burnt gases occurs with which, the return impact occurs is influenced within limits by a variation in length of the exhaust pipe.

Figure 6 illustrates the influence of a variation in length of the exhaust pipe upon the In this figure three curves are shown which are all taken at the same engine speed, but with exhaust pipes of different lengths fitted to the engine, the length of the exhaust pipes being 2 feet 6 inches (chain dotted curve), 4 feet 5 inches (full line curve), and 5 feet 8 inches (dotted curve), respectively.

The curves shown in Figure 6 are similar to those represented in Figures 1 to 5, but indicate pressure variations only.

It will be seen that as the length of the pipe increases, the return impact becomes more distant, a longer period of time is available for 76 charging and consequently the effect of the return impact upon the cylinder is reduced in intensity. a

If the length of the exhaust pipe continues to be increased, a point is reached when no further retardation of the return impact can be obtained.

If the exhaust pipe is of tapered form flaring suitably outwardly, then a further increase beyond this point will not be objectionable, since the continued increase in cross section of the exhaust pipe will permit the expansion of the gases to occur, but such an increase in length will be unnecessary. If, on the other hand, the exhaust pipe is cylindrical in form, then by lengthening the exhaust pipe still further after the point of maximum retardation has been reached, the return impact will again commence to occur sooner and this is due to the fact that the cylindrical form of the pipe will not permit the free expansion of the gases and produces a choking effect which opposes the final exit of the gases after their expansion.

If a silencer or expansion chamber is fitted on the exhaust pipe, then in the case when the exhaust pipe is of taperedform, such a silencer may be fitted at the end of an exhaust pipe of substantially the required length to give the maximum retardation in the return impact, but as will be seen from the above remarks, the silencer may be fitted at the end of a suitably tapered exhaust pipe of more than the necessary length without objection.

In the case when the exhaust pipe is cylindrical in form, the silencer should be'fitted at the end of an exhaust pipe of substantially the required length to give the maximum retardation, and if any further lengthening of the exhaust pipe is required, this should be provided beyond the silencer or expansion chamber.

' By way of indication it may be stated that in general it will be found that a length of exhaust pipe from 3 feet to 6 feet will be found suflicient for the above purposes.

In explanation of the above it may be stated that the volume of the void left in the cylinder and the exhaust pipe by the mass exit of the burnt gases will depend upon the mass of the external gaseous medium that can be displaced by the work the exhaust gases are allowed to do by impact upon this external medium, other things being equal.

The time absolute for which this void lasts is dependent upon the length of the path travelled by the exhaust gasesbefore they rebound towards the cylinder, or more generally upon the velocity of exit of the burnt gases and the negative acceleration they then undergo.

If the exhaust gases are allowed to issue directly from the exhaust orifice into the open atmosphere, the head of the issuing column of gases will be flattened and enlarged. The resisting surface will thereby be increased in area and the negative acceleration applied to the issuing mass will be extremely high. Consequently the path travelled by the gases from the cylinder will be extremely short and the rebound into the cylinder will follow with extreme rapidity.

If on the other hand the gases have to pass through an exhaust pipe or the like before reaching the open atmosphere, and this pipe is too' formed against the atmosphere external to the pipe so that the time elapsing between the exit and the return of the exhaust gases is reduced in relation to the extent of this deformation of the column.

If the exhaust pipe is lengthened, a dimension will be reached for which for a given explosive force the rebound of the burnt gases will occur from a plane or frontal zone situated adjacent the end of the pipe. After this length of the pipe has been reached, the further lengthening of the pipe will not produce any additional delay in the occurrence of the return impact. In other! words, there will be no advantage in lengthening the pipe beyond this point.

If silencers or expansion chambers are provided on the exhaust duct, their position will be determined exactly by the point from which the rebound of the burnt gases occurs for the strongest explosions.

At explosions of high intensity the energy contained in the burnt gases and their exit velocity are greater and consequently the point from which they rebound is situated more distant from the engine than for explosions of lower intensity.

For weak explosions, therefore, everything will occur as if the exhaust pipe were too long, in other words, without advantage from the point of view of the present invention.

Expansion chambers will however become objectionable if they are situated too near the cylinder, that is to say, nearer the cylinder than the furthest point of rebound of the column of exhaust gas for the strongest explosion over the working range of the engine.

Such a sudden increase in section of the exhaustpipe as a consequence of the provision of an expansion chamber or the like, nearer than the point of rebound of the burnt gases, would have a similar objectionable effect to that which occurs if the point of rebound is allowed to become situated in the open atmosphere. In-other words, the point of rebound would become formed in the expansion chamber itself, the return would consequently occur in a reduced space of time and the period available for charging would be shortened.

According to the invention therefore, the characteristics of the exhaust system are so related to the energy contained in the burnt gases upon their exit from the cylinder at maximum intensity of explosion that the return impact of the said burnt gases always occurs from a point substantially within the said exhaust system, the shape and form of the said exhaust system being such that it comprises no sudden changes in section and that the issuing mass of burnt gases is always thereby guided and directed away from the cylinder.

This result may be attained by providing the engine with an exhaust pipe of such a length that the return of the burntgases for an explosion of maximum intensity occurs from a point situated substantially within the said exhaust pipe, and the requisite length of such an exhaust pipe can readily be determined by trial, as will be understood from the foregoing.

Further, when the engine comprises a silencer or expansion chamber according to the invention, such silencer or expansion chamber will be situated on the exhaust system or duct at a point more remote from the cylinder than the point from which the return impact of the burnt gases occurs.

The above considerations concerning the form and length of the exhaust passages and ducts apply equally well to a single cylinder and multicylinder engine, but in determining the form and arrangement of the exhaust ducts and manifolds for a multi-cylinder engine further considerations arise as will be understood from the following.

In aninternal combustion engine according to the invention, normally the exhaust, the passage of fresh gases through the cylinder and the end of the charging will occupy about 120 of the crank movement.

According to the relative timing of the cylinders, the total phases of exhaust and charging may overlap.

It may occur that the exit from one of the cylinders is produced at the same time as an admission occurs into another cylinder and according to the number of cylinders the phases of admission and exhaust may themselves over-- lap.

Normally for three cylinders grouped together on a crank shaft, these total phases of outlet of the burnt gases and entry of the fresh charge are separatedin time or overlap very little according to the timing of the engine.

With the timing represented in Figures 1 to 5, for example, in a three cylinder engine there will be an overlap of a few degrees.

In engines with four or more cylinders all these phases overlap.

When establishing the exhaust manifolds for a multi-cylinder engine, care must be taken to,

establish a protectionagainst disturbances that may be produced by the exhaust from one of the cylinders upon another cylinder.

The burnt gases upon their mass exit from the cylinder and also upon their rebound towards the cylinder, behave like a projectile and are governed by the laws of reflection.

Consequently if the burnt gases upon their outward passage from one cylinder encounter surfaces which tend to reflect them towards-another cylinder in which the exhaust and inlet orifices are open at this moment, this may have an objectionable effect upon the charging of the latter. 4

Consequently in arranging the junctions between the ducts and manifolds in a multi-cylinder engine, care will be taken to ensure that the walls of the ducts and junctions are profiled in such a way that the angles of reflection always lead the gases to the exterior and away from.

another cylinder which may be open at this moment.

Further, in establishing the exhaust manifolds and exhaust pipes, the same rules 'must be observed as for an exhaust pipe of a single-cylinder engine, that isto say, there must be no sudden and considerable increase or decrease in cross section on the path followed by the gases as this will cause the exit speed to be reduced and a total or partial rebound may be produced which may influence the cylinder which is exhausting or one of the cylinders in which they exhaust and inlet orifices are open.

In addition when the mass of burnt gases leaves the cylinder during the exhaust period, and'when it is passing a junction whicht is also connected with a cylinder open to admission at this moment, and in which exhaust is still open, the mass of burnt gases inpassing the junction causes a shock to be transmitted along the exhaust duct leading from the second mentioned cylinder and this shock may have an objectionable repercussion on the admission of the charge into the-latter cylinder.

This shock will be transmitted for a certain distance, after which its effect will be no longer noticeable. This objectionable effect maybe overcome by making the distance between the junction and the second cylinder of a length at "least equal to the minimum distance that will ensure that the objectionable effect mentioned I above will not be transmitted to this cylinder.

A suitable distance for any particular case will be found by trial, but as a general guide it may be mentioned that adistance of 20 to 30 cms. will generally be suflicient for an engine of 1 litre capacity per cylinder.

Alternatively, reflecting baflies of the form described and shown in application Serial No. 738,016 filed August 1, 1934 may be provided at the junctions between the ducts leading from two cylinders, in order that the effect of any such shock will be minimized.

Care should also be taken to combat any objectionable action which may be produced by a prolonged suction caused by the exit of the burnt gases from one cylinder upon the charging of another cylinder. This .maybe avoided by a suitable closure of exhaust, as described in application .Serial No. 84,184 filed June 8, 1936.

Further, the return of the burnt gases which have left one cylinder may have an objectionable effect upon the charging of another cylinder and a protection should be afforded against such objectionable action.

This protection may also consist in establishing a suitable closure of exhaust as described in application Serial No. 83,120 filed June 2, 1936 for the cylinder to be protected or by the provision of the reflecting baliles of application Serial No. 738,016 referred to above which redirect any returning gases in the direction of exhaust or by any other suitable means which ensure that the said objectionable action will be combated.

In establishing the junctions between the exhaust ducts and manifold of two cylinders or of two groups of cylinders, account must also be taken of the state of the gases at the junctions and the ducts which will be traversed by the exhaust gases. Two extreme conditions may be produced. Either the exhaust gases issuing from one cylinder may encounter the return impact of the preceding exhaust or the exhaust gases may enter a void left by the preceding exhaust.

If it is desired to reduce the absolute time of exhaust, an arrangement may be adopted such that each exhaust from one cylinder falls into the void left by exhaust from the other cylinder. The same considerations apply to the columns of exhaust gases, and the columns of entering gases which follow the exhaust columns. Those columns which have the same direction of movements do not exert any resistance upon each other, on condition that the spaces and the sections of passage permit them to intermingle without too much deformation, and it is logical to,

consider that the angles at which such columns encounter each other must be reduced to a minimum.

It should be noted that with a multi-cylinder the cylinders adjacent the cylinder which is under exhaust.

2. That exhaust gases moving outwards through the piping may encounter returning gases from the preceding exhaust and may arrest these returning gases at a constant distance from the cylinders.

By means of the invention, the cylinders may be protected against any disturbances from the adjacent cylinders by the length of the first junction, by the surfaces which reflect and guide the gaseous column, by the sections permitting the free passage of the columns without restriction and without crushing against the ambient mass, by the position of the junction of the manifolds.

This position is determined by the position of the returning column of gases from the preceding exhaust, the length, the volume of the exhaust duct after these first junctions and the position and volume of the silencer or expansion chamber.

The above remarks will be more clearly understood by referring to Figure 13 which is a timing diagram for a six-cylinder engine, each cylinder of which has the inlet and exhaust events established as indicated in Figures 1 to 5. In this figure it is assumed that the firing order is 1-53-6--24 by way of example. The figure shows simply the 60 intervals between the exhaust openings and the order in which such openings occur.

It will be seen from this figure and by referring to Figures 1 to 5 that the exhaust from one cylinder always occurs in the middle of the admission period of the preceding cylinder and may have a repercussion on this admission.

By making a manifold for each group of three cylinders 1, 2, 3 and 4, 5, 6, and extending these manifolds a suificient distance before connecting them together as indicated in Figures 14 and 15, it will be easy to ensure that the above objection will be avoided.

But by referring to Figures 1 to 5 it will also be seen that at all speeds above 800 R. P. M., the return of the burnt gases following the exhaust from one cylinder, occurs after the opening of the exhaust of the succeeding cylinder.

If the two manifolds are extended to form separate exhaust pipes or if the junction between them is more remote from the cylinder than the point from which the return takes place, then no advantage can be obtained from the fact that the exhaust .from a cylinder in one manifold occurs before the return towards the preceding cylinder in the other manifold. A

On the other hand, if the manifolds are connected to a common exhaust pipe as shown in Figure 15 and the junction between these manifolds is situated nearer the cylinders than the point in the piping from which the nearest return "of the burnt gases occurs then the exhaust gases from one cylinder, say cylinder I, will pass this junction and will not have commenced to return at the moment when, the exhaust of the next cylinder, say cylinder 5, commences so that the exhaust from the latter cylinder will enter piping which is under depression as stated above.

These exhaust gases from the cylinder 5 will pass the junction and will oppose the returning exhaust gases from cylinder I, so that when the next cylinder, say cylinder 3, opens the exhaust gases from the latter will also enter a depression in the piping and so on.

At high speeds the effect of this interaction between the exhaust gases may be such as to annul the effect of the return of the exhaust gases completely.

Moreover, and as explained in connection with the exhaust pipe of a single cylinder engine, the length of exhaust pipe following the junction between the manifolds should be sufficient to ensure that the return of the burnt gases occurs from a point situated within this pipe, whereby the return of the burnt gases will be delayed as much as possible and if a silencer or expansion chamber is fltted it will be fitted on this pipe, not nearer the cylinder than the point from which the furthest return occurs for a maximum intensity of explosion.

In practice the cylinders of a six or eight cylinder engine may be divided into two groups, the cylinders of each group exhausting into a common manifold. These two manifolds may extend over a distance approximately equal to the length of the engine itself and thereafter connected together so as to lead into a single exhaust pipe upon which is mounted a silencer d8.- vice common to all the cylinders.

A suitable construction and arrangement of the exhaust piping of a six cylinder engine having the above timing and constructed in accordance with the above modifications is illustrated in Figures 14 and 15.

Cylinders I, 2 and 3 are connected to one manifold '6 and cylinders 4, 5 and 6 to the second manifold I and these manifolds are connected at 8 to a simple exhaust pipe 9 upon which is mounted a silencer l0.

It will be seen that the main walls of all the passages are so formed that they tend to guide and direct the exhaust gases towards the exterior and so that they do not obstruct or restrict the outward passage of the gases.

It will also be understood that in this example, if the junction 8 is sufficiently remote from the cylinders to protect the charging of cylinder 3 from a repercussion of the exhaust from cylinder 6, this will ensure that all the other cylinders are sufficiently protected in the same manner.

It will also be understood that the junction 8 must be nearer the cylinders than the point from which the return of the burnt gases occurs, and

'that the silencer ill must be more remote from the cylinders than this point.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

1. Method of controlling variable speed two stroke cycle internal combustion engines, which comprises establishing communication between the cylinder and exhaust system during the firing stroke, providing at all engine speeds over a chosen speed range, for the issuance of the burnt gases from the cylinder substantially as a mass in an interval of time shorter than that which would be required for the burnt gases to expand down to the ambient pressure by adiabatic flow, whereby the mass of gas moves outward and thereafter returns from a point which may be within the exhaust system, providing a permanent free passage for the burnt gases to the limit of outward travel of said burnt gases, preventing at the highest engine speed the entrance of fresh charging air until the said issuance of the burnt gases is in full progress, admitting fresh charging air into the cylinder, at the highest engine speed, when the said issuance of the burnt gases is in full progress and causes a suction effect to be exerted in the cylinder, thereby ensuring that at all lower engine speeds the said admission will not commence until the said issuance of the burnt gases has occurred, and providing at all engine speeds for the said fresh charge to occupy the cylinder and a portion of the exhaust system in the interval elapsing between the said exit of the said gases and the instant when the pressure of the returning gases becomes effective within the cylinder.

2. Method of controlling variable speed multicylinder two stroke cycle internal combustion engines, which comprises establishing communication between each cylinder and its exhaust system during the firing stroke in said cylinder, providing at all engine speeds over a chosen speed range for the issuance of the burnt gases from each of said cylinders substantially as a mass in an interval of time shorter than that which would be required for the burnt gases to expand down to the ambient pressure by adiabatic flow, whereby the mass of gas moves outward and thereafter returns from a point which may be within the exhaust system, providing a permanent free passage for the burnt gases issuing from each cylinder to the limit of outward travel of said gases, providing a communication between a plurality of the said free passages, and providing for the said burnt gases issuing from one cylinder to encounter in its free passage a depression caused in this passage by the issuance of burnt gases from another cylinder, preventing at the highest engine, speed the entrance of fresh charging air into each cylinder until the said issuance of the burnt gases is in full progress, admitting fresh charging air into each cylinderat the highest engine speed when the said issuance of the burnt gases is in full progress and causes a suction effect to be exerted in the cylinder, thereby ensuring that at all lower engine speeds the said admission will not commence until the said issuance of the burnt gases has occurred, and providing at all engine speeds for the said fresh charge to occupy the cylinders and a portion of the exhaust systems of said cylinders in the interval elapsing between said exit of the burntgases and the instant when the pressure of the returning gases becomes eifective within the cylinders.

3. A variable speed multi-cylinder two stroke cycle internal combustion engine having exhaust and inlet orifices on the cylinders and an exhaust conduit on the said orifice of each cylinder, the said exhaust conduits being connected together in groups to a common manifold, means for so controlling the exhaust orifices of the several cylinders as to ensure the issuance of the burnt gases substantially as a mass at all engine speeds above a chosen maximum speed, whereby the said mass moves outward and thereafter returns toward the cylinder, means for so controlling the several inlet orifices as to ensure, at the highest engine speed, that the said inlet orifices will be opened while the corresponding exhaust orifice is still open and when the said issuance of the burnt gases is in full progress and produces a suction effect in the cylinder, the said conduits and manifolds providing a permanent free passage for the burnt gases from the several cylinders to the limit of outward travel of said gases, the several conduits being connected together in a group having a suflicient length and being so formed internally at their connection to the manifold as to provide a passage which guides the burnt gases away from any other cylinder of the group which is open at the same time, and to protect the said other cylinder from an objectionable action by said issuing gases.

4. An engine as claimed in claim 3, a plurality of manifolds being connected together to deliver into a single exhaust pipe, the said connection being established at a point situated nearer the several cylinders than the shortest limit of outward travel of the burnt gases from any of said cylinders.

MICHEL KADENACY. 

